1. Field of the Invention
This disclosure concerns optical devices involving a photoluminescent materials such as quantum dots within or on top of a retroreflector and methods of using them for safety and design.
2. Description of the Prior Art
Retroreflective materials, or materials that reflect light back to the source, have been around since the early 1900's when road safety reflectors were used on vehicles such as horse-drawn buggies, cars, bikes, and motorcycles before finding demand in improving pedestrian visibility.
The first patent for a reflector was granted to Rudolf Straubel in 1906 (U.S. Pat. No. 835,648 A). Straubel teaches that his invention was an improvement of Beck's “triple mirror” system and did not describe any specific uses. In 1912, Robert Venner patented a method of increasing the visibility of signs (U.S. Pat. No. 1,048,008 A) using partially transparent marble-sized glass beads, also known as cataphotes or “cat's eyes”, or grooves arranged as to give the effect of luminous letter or images when illuminated from behind. Venner invention was used widely in the 1920s in Great Britain in order to increase the visibility of traffic signs. Jonathan Stimson was granted the first patent for a “triple reflector” in 1923 (U.S. Pat. No. 1,743,834 A) which could be grouped together could give the effect of luminous designs on safety signs (U.S. Pat. No. 1,809,761 A) when illuminated from the front.
In the 1920's and '30s, the growth in car and bicycle usage drew increasing attention to traffic safety. In Europe the first traffic laws were going into effect and some of those included mandatory bike reflectors. In 1933, Branson Edward H was awarded a patent for road signs incorporating cataphote reflectors (U.S. Pat. No. 2,071,294 A) that were intended to make warning signs more visible at railway crossings. That same year Great Britain installed cataphotes in some roadways. In 1949, General Motors received a patent for a combination of plastic reflector with a car tail light (U.S. Pat. No. 2,685,231 A). What made these developments distinguishing was the specific utilization/reflection of light incident from automobile headlights.
The first mention that pedestrians should also attempt to make themselves more visible on roadways, was published in the January 1943 issue of Popular Science. There is no record of any invention by US highway police officer Raymond Trask, but the Popular Science article describes his “red reflectors glow at car's approach”. This is also the first known report of a color-filtered reflecting device but it was not clear how the red color was to be produced. Widespread use of pedestrian reflectors notably occurred in Finland in the 1960s, with the development of plastic prism reflectors. Through various public campaigns, reflector use by Finish pedestrians had reached approximately 40% by the end of the decade while pedestrian traffic accents fell by about 80%.
In 1958, Alfred Nellessen patented retroreflective coatings containing tiny glass beads (U.S. Pat. No. 3,099,637 A) that could make essentially any ordinary surface reflect light back towards the source, specifically with car headlights in mind. This technology is now widely used in road markings and signage. Although adoption of these technologies increased dramatically over the years, few improvements have been made. Current generations of road safety retroreflectors fall into two categories; 1) plastic cube corner prism reflectors that can be colored by color-filtering pigments and 2) coatings that contain microspheres that sit on top of or underneath traditional paint of any color.
The current generation of retroreflectors have two major drawbacks that limit their utility. First, all reflective coloring results from absorptive filtering that waste incident light. For example, a red plastic bike reflector absorbs all visible colors except red, and reflects the red light. The blue, green, yellow, and other spectral components of incident headlights are absorbed and lost when incident on a red bike reflector. Second, retroreflectors work very well at reflecting directly to the light source, but the light source is not necessarily close to the eyes of an observer. A particularly problematic example is with large trucks where the headlights can be several meters from the line of sight of the driver. A photoluminescent retroreflector may solve both problems by down-converting light to a luminous glow that would have been thrown away by color-filtering.
Photoluminescence (PL) is the emission of light (electromagnetic radiation, photons) after the absorption of light. It is one forms of luminescence (light emission) and is initiated by photoexcitation (excitation by photons). Following photon excitation, various charge relaxation processes can occur in which other photons with a lower energy are re-radiated. Time periods between absorption and emission may vary: ranging from short femtosecond-regime for emission involving free-carrier plasma in inorganic semiconductors up to milliseconds for phosphorescent processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours.
Colloidal semiconductor nanocrystals, commonly known as quantum dots (QDs), are known for their size-tunable optical properties, including PL, and their inexpensive processability from liquids. In particular, they are very effective at absorbing a broad spectrum of light and then converting that energy into emitted light of a single color that is determined by their size. Optical properties (absorption and emission spectra) can be programmed in by tailoring the manufacturing conditions to realize different sizes, shapes, and/or compositions. This fundamental property of QDs has spurred research and development of fluorescence biolabeling, color-specific light-emitting-diodes, and vibrant displays. However, the current generation of QDs are toxic and far too expensive to reach most markets. There is a unique opportunity for QDs that are both low-cost and non-toxic as active elements of luminescent composites for improved road safety (e.g., making objects more visible) and design (e.g., eye-catching fluorescent pigments).
Nanocrystal quantum dots of the I-III-VI class of semiconductors, such as CuInS2, are of growing interest for applications in optoelectronic devices such as solar photovoltaics (PVs, Stolle, C. J.; Harvey, T. B.; Korgel, B. A. Curr. Opin. Chem. Eng. 2013, 2, 160) and light-emitting diodes (Tan, Z.; Zhang, Y.; Xie, C.; Su, H.; Liu, J.; Zhang, C.; Dellas, N.; Mohney, S. E.; Wang, Y.; Wang, J.; Xu, J. Advanced Materials 2011, 23, 3553). These quantum dots exhibit strong optical absorption and stable efficient photoluminescence that can be tuned from the visible to the near-infrared (near-IR, Zhong, H.; Bai, Z.; Zou, B. J. Phys. Chem. Lett. 2012, 3, 3167) through composition and quantum size effects. In fact, Gratzel cells sensitized by specifically engineered CISeS quantum dots have recently been shown to offer excellent stability and certified power conversion efficiencies of >5%. (McDaniel, H.; Fuke, N.; Makarov, N. S.; Pietryga, J. M.; Klimov, V. I. Nat. Commun. 2013, 4, 2887.) CuInZnS2 QDs (CuInS2 alloyed with ZnS) are particularly attractive for luminescent retroreflectors because of their large intrinsic Stokes shift (separation between absorption and emission of the QDs) that prevents self-absorption of the PL. In the luminescent retroreflectors disclosed herein, the luminescence must be able to pass through the luminescent element without being re-absorbed which leads to significant losses, similar to what occurs in luminescent solar concentrators (Meinardi, F., Colombo, A., Velizhanin, K. A., Simonutti, R., Lorenzon, M., Beverina, L., Viswanatha, R., Klimov, V. I. & Brovelli, S. Nat. Photon. 2014, 8, 392.).
Another prior art is the J. Wallace Parce patent for QD-doped matrixes for use as down-converting layers (U.S. Pat. No. 7,374,807 B2). The Parce patent does describe “polymeric layers [containing nanocrystals] used to coat optical devices (e.g., refractive lenses or reflective elements)” but not retroreflective elements for safety or design applications, and they explicitly left out enabling I-III-VI QDs. Typical QDs have a small Stokes shift (separation between absorption and photoluminescence) which causes the emission to be self-absorbed. Attempts to solve this problem with thick shells, of CdS for example, has led to a big variation between the absorptive pigment of the composite from the luminescence. In other words, CdSe/CdS QDs generally appear yellow (due to absorbance coming from yellow CdS) but their PL is red (coming from red-emitting CdSe). Furthermore, the invention herein overcomes the toxicity and cost problems with previous generations of QDs enabling safety and design uses of never previously imagined luminescent retroreflectors.